This invention relates to dialysis and hemodiafiltration in general and, more particularly, to improved hemodiafiltration methods and devices for removal of blood toxins.
Hemodialysis and Hemodiafiltration are well known methods for removing toxic substances from a patient""s blood, thereby reducing the level of toxins in the patient""s blood as part of an extracorporeal blood cleansing system. Both these methods are based on flowing blood through a cartridge containing a semi-permeable membrane which separates the cartridge into two compartments. In general, hemodialysis is a process whereby blood flows through a blood-side compartment of the cartridge, while a cleansing solution, i.e., a dialysate solution, flows through a dialysate-side compartment of the cartridge. Toxins are removed from the blood by diffusion across the semi-permeable membrane from the blood-side compartment to the dialysate-side compartment. The rate of diffusion is determined by the concentration gradient established between a higher concentration of toxins in the blood relative to the dialysate fluid. Hemodiafiltration is process whereby the normal removal of toxins by diffusion is augmented by a convective flow of plasma water across the semi-permeable membrane which assists in carrying toxins by bulk flow of fluid from the bloodside of the membrane to the dialysate side of the membrane. The transportation of plasma water across the semi-permeable membrane is achieved by establishing a pressure gradient, generally referred to as Transmembrane Pressure (TMP), across the membrane. In hemodiafiltration, an equivalent amount of a substitution fluid, or replacement fluid, must be added to the blood to replace the plasma water that is filtered across the membrane. This substitution fluid is generally added either before the blood enters the cartridge (pre-dilution mode) or after the blood exits the cartridge (post-dilution mode).
Hemodiafiltration systems using two cartridges connected in series are also known in the art. In such systems, a first cartridge is used as a conventional diafiltration cartridge providing simultaneous diffusion and filtration of plasma water across the semi-permeable membrane. In a second cartridge, toxins are diffused from the blood to the dialysate fluid, and a reverse pressure gradient is used to reverse-filter dialysate fluid from the dialysate-side compartment, across the membrane, and into the blood-side compartment. The reverse-filtered dialysate fluid serves as a substitution fluid to replace the amount of plasma water that filtered from the blood-side compartment to the dialysate-side compartment in the first cartridge. Such a method is described in J. H. Miller et al., xe2x80x9cTechnical Aspects of High-Flux Hemodiafiltration for Adequate Short (Under 2 Hours) Treatment,xe2x80x9d Transactions of American Society of Artificial Internal Organs (1984), pp. 377-380.
It is an object of this invention to provide a method and a system using two dialyzes or diafiltration cartridges, or two stages of a single dialyzer cartridge, whereby the temperature of the dialysate fluid entering the first cartridge is increased above the normal dialysate temperature. The increased temperature increases the solute diffusivity, or diffusion coefficients, thereby enhancing the removal of substances by diffusion in the first dialyzer stage. Another advantageous result of increasing the dialysate temperature is that the viscosity of the blood is decreased, thereby enhancing the filtration of plasma water across the semipermeable membrane for a given transmembrane pressure in the first dialyzer stage. This enhances the removal of solutes by convection as the increased bulk flow of plasma water carries additional toxic substances. In the second dialyzer stage, e.g., a second dialyzer cartridge, the blood temperature is decreased back to its normal level, e.g., by heat exchange and/or by addition of substitution fluid at normal blood temperature.
The present invention provides a hemodialysis or hemodiafiltration system and method using two dialyzer stages, e.g., two dialyzer cartridges connected in series, wherein the temperature of the dialysate fluid stream of the first dialyzer is increased such that the blood in the first cartridge is dialyzed or diafiltered at an elevated temperature, while blood in the second cartridge is dialyzed or diafiltered against a dialysate stream at normal blood temperature (i.e. at approximately 35xc2x0 C. to 40xc2x0 C.). The second cartridge may serve to correct for blood temperature shifts caused by high dialysate temperature in the first cartridge, and to further remove blood toxins by diffusion or diafiltration against a at normal blood temperature. In hemodialysis applications, transfer of heat from the higher temperature blood to the lower temperature dialysate in the second cartridge may assist in correcting (i.e., lowering) the blood temperature. In hemodiafiltration applications, the blood temperature may be lowered before the second dialyzer stage by adding substitution fluid into the blood stream.
The present invention may be embodied in an improved dialysis machine that includes means for heating a portion of the dialysate fluid path above normal blood temperature, e.g., above 40xc2x0 C. Such a machine may include other components used in dialysis machines, such as a water preparation module to degas and heat water necessary for preparing dialysate, an ultrafiltration control system that may include a flow balancing system and an ultrafiltration (UF) pump, a dialysate proportioning system that introduces dialysate concentrates into the water stream, and an extracorporeal monitoring and control devices which may includes a blood pump for circulating blood through an extracorporeal circuit.
In some embodiments of the invention, the system is also designed to perform hemodiafiltration, and additional components may be used. For example, the system may include a substitution fluid supply system including a pump and substitution fluid filters for preparing substitution fluid xe2x80x9con-linexe2x80x9d using a supply of dialysate fluid. The system may also include an interdialysate flow control system, which may including interdialysate pump, to regulate the relative ultrafiltration rates of the two dialyzer cartridges.
A hemodialysis/hemodiafiltration system and method using two cartridges connected in series is disclosed in PCT Application No. PCT/US99/25804 entitled xe2x80x9cNon-Isosmotic Diafiltration Systemxe2x80x9d filed in the name of Collins et al., the entirety of which is incorporated herein by reference. In the system of Collins et al., the dialysate fluid in the first cartridge is made hypertonic or hypotonic, by appropriately adjusting the electrolyte levels of the dialysate stream, to improve toxin removal efficiency.